The present invention relates to vapor compression refrigeration systems having tube in shell heat exchangers and, more particularly, relates to vapor compression refrigeration systems having tube in shell subcoolers used in low temperature cooling applications such as in a brine chilling application.
It is well known that use of a subcooler with a standard cycle vapor compression refrigeration system reduces the refrigeration system compressor power requirements thereby improving the operating efficiency of the refrigeration system. However, in low temperature cooling applications such as a brine chilling application where brine is used as the heat exchange medium chilled in the evaporator of the vapor compression refrigeration system, tube in shell subcoolers have not been used because of the danger of freezing water, which is used in the tubes of the subcooler as a heat exchange medium, at shutdown of the refrigeration system. Water freeze-up is a problem because usually the brine in the evaporator is chilled in the evaporator to a temperature below the freezing temperature of water and equalization of refrigerant temperature and pressure in the refrigeration system at shutdown of the refrigeration system results in refrigerant temperatures in the subcooler dropping to a temperature below the freezing temperature of water for a period of time sufficient to freeze the water in the tubes of the subcooler. If too much water freezes in the tubes of the subcooler, then the subcooler tubes may break or other such undesirable results may occur.
The danger of freezing water in the tubes of the subcooler may be reduced by taking measures such as maintaining water flow through the tubes of the subcooler after shutdown of the refrigeration system for a period of time sufficient to allow the refrigeration system to equalize at a temperature above the freezing temperature of water. However, this type of operation is inefficient because a water pump must be maintained in operation for a period of time after each shutdown of the refrigeration system. Also, if the water pump should fail, or if there is a power failure for the refrigeration system, then subcooler freeze-up may still occur. Therefore, this is not a particularly satisfactory way of overcoming the subcooler freeze-up problem.